Asphalt is typically produced in a drum by heat drying virgin aggregate and by adding to it and mixing with it liquid asphalt cement, fillers and other additives, often including reclaimed or recycled asphalt pavement (commonly referred to as RAP). Recycled and reclaimed asphalt pavement are commonly understood to be the same.
Typically, the virgin aggregate is introduced and heated in a direct fired dryer using super heated gasses and direct radiation from a burner flame, such as a counter-flow drum. This method is sufficient for virgin aggregate because no significant undesirable fumes are generated from heating the virgin aggregate as quickly as possible. Heating the RAP under similar conditions, in contrast, often generates undesirable fumes such as gaseous hydrocarbons and other chemicals and particulates. In many cases, if the RAP is heated in the direct fired dryer using direct radiation from the burner flame, visible smoke from the RAP is emitted from the asphalt plant.
In order to separate and minimize the undesirable fumes, others have utilized a mixing and drying drum or a large single drum and allowed the heated virgin aggregate to heat the RAP. In this approach, the RAP is introduced downstream of the volume heated by the burner head. The heated virgin aggregate heats the RAP without the use of direct radiation from the burner flame. This process may be acceptable in circumstances such as when the RAP has sufficiently low moisture content to permit it to be completely heated and dried in the processing time allowed for in a standard drum unit.
Another approach to heating the RAP is to build the asphalt plant with a recycle feed assembly that introduces the RAP into the drum downstream of the burner head. The RAP passes within the direct heat from the burner. However, depending upon the moisture content of the RAP, smoke and other undesirable gases and fumes may be created.
In general, if the RAP is naturally dry, it is desirable to introduce it into the mixing drum without exposure to the high heat of the burner flame. On the other hand, if the RAP has a high moisture content, then the RAP can withstand the direct heat to dry off the moisture efficiently; specifically, so long as the RAP is moist, it will not smoke when heated.
A need exists for a system permitting the adjustment of the location of the RAP feeder and/or RAP inlet to the drum. Adjusting the location of the RAP feeder and/or inlet permits the operator of the plant to tailor the unit to physical characteristics of the RAP such as moisture content, gradation of the RAP or other environmental factors. A further need exists for a system permitting the adjustment of the location of the combustion volume of the burner. A need also exists for a method of reducing the emissions during the operation of an asphalt plant.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior dryers of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.